1. Field Of The Invention
The present invention relates to a particle detecting method which projects a light beam on the surface of a workpiece and determines the position of a minute dust particle on the surface of the workpiece through the observation of changes in the light beam caused by the minute dust particle. The present invention also relates to a particle detecting system which carries out the above particle detecting method.
2. Discussion Of The Background
Defects attributable to dust particles adhering to semiconductor wafers are principal factors which reduce Integrated Circuit (IC) yield in the manufacture of Very Large Scale Integrated (VLSI) circuits, such as 16-Mb DRAMs. Although some minute dust particles may have not been causes of problems in the past, those minute dust particles introduced in the manufacturing process adhere to wafers and have become sources of contamination with the progressive reduction of the width of lines forming patterns. Generally, the size of minute dust particles which will cause problems is a fraction of the minimum width of lines of a VLSI circuit to be fabricated. It is a generally accepted idea that minute dust particles of diameters on the order of 0.1 .mu.m can not be ignored when fabricating 16-Mb DRAMs, in which the width of the narrowest lines is 0.5 .mu.m. Such minute dust particles contaminate VLSI circuits, causing the disconnection of circuit patterns and short-circuiting, make VLSI circuits defective and reduce the quality and the reliability of VLSI circuits. Accordingly, it is a key to the improvement of IC yield to detect minute dust particles, to measure and grip accurately and quantitatively the actual state of existence of dust particles on workpieces, and to control dust particles.
A dust particle inspection system capable of detecting dust particles on the surface of a flat workpiece, such as a silicon wafer, and determining the positions of the dust particles has been used for inspecting workpieces. A dust particle detecting method by which a conventional dust particle inspection system detects dust particles will be described hereinafter.
A light-scattering dust particle detecting method is employed by a dust particle inspection system for detecting dust particles. The light-scattering dust particle detecting method scans the surface of a wafer with a light beam, measures the variation of intensity of scattered light with time linearly by a photo-multiplier tube, and detects a dust particle and determines the position of the dust particle from the relation between a moment when a scatter signal is generated upon the reception of scattered light scattered by a fine particle and the position of the scanning light beam on the surface of the wafer at the same moment. Dust particle inspection systems IS-200 and LS-6000 available from Hitachi Electronics Engineering Ltd., Surfscan 6200 available from TENCOR, and dust particle inspection system WIS-9000 available from ESTEK are known as dust particle inspection systems. Measuring principles on which those known dust particle inspection systems operates and the configuration of those dust particle inspection systems are explained in, for example, "ANALYSIS AND EVALUATION TECHNIQUE FOR HIGH PERFORMANCE SEMICONDUCTOR PROCESS", by Semiconductor Basic Technology Research, REALIZE INC., pp. 111-129.
The accuracy of measurement of fine particles of the conventional measuring method using scattered light is limited by noise generated in a measuring system and included in a scatter signal representing light scattered by fine particles. Noise attributable to the surface roughness of a silicon wafer, which is called haze, makes the detection of fine dust particles of 0.10 .mu.m or below in particle size on the surface of the silicon wafer very difficult. This problem is explained in detail in, "SEMICONDUCTOR MEASUREMENT AND EVALUATION HANDBOOK", by SCIENCE FORUM, pp. 474-479. However, there has not been established any method of detecting minute dust particles of 0.07 .mu.m, 0.04 .mu.m and 0.03 .mu.m in particle size which must be controlled in the manufacture of VLSI circuits, such as 64-Mb, 256-Mb and 1-Gb DRAMs, although those VLSI circuits are expected to be developed and mass-produced in the future, in which the width of the narrowest lines is 0.35 .mu.m, 0.20 .mu.m and 0.15 .mu.m, respectively.
A fine particle measuring method using the scattering of light, which is carried out by the conventional dust particle inspection system, scans the surface of a workpiece, such as a silicon wafer, with a light beam, and detects the variation of the quantity of scattered light linearly by a photodetector, such as a photo-multiplier tube. Therefore, the measured position of a minute dust particle includes an error corresponding to the area of a pixel dependent on the area of a spot formed by the light beam on the surface of the workpiece, and hence a precise determination of the position of the dust particle is impossible. The light beam must be focused on the surface of the workpiece in the smallest possible spot to achieve the highly accurate determination of the position of a dust particle on the surface of the workpiece. However, there is a limit to the reduction of the spot of the light beam. If the light beam is focused in a very small spot, the total length of scanning lines for scanning the entire surface of the workpiece increases, and an increased measuring time is necessary. Usually, the pixel of the current system is 20.times.200 .mu.m.sup.2. The area of a focused laser beam used by the conventional dust particle inspection system is explained in detail in "ANALYSIS AND EVALUATION TECHNIQUE FOR HIGH PERFORMANCE SEMICONDUCTOR PROCESS", by Semiconductor Basic Technology Research, pp. 111-129.
When detecting a minute dust particle of 0.10 .mu.m or below in particle size, the minute dust particle must be detected at a high sensitivity and at a high S/N ratio, and the position of the dust particle must be determined in a high accuracy. Dust particle detecting methods disclosed in Japan Laid Open Patent Publications (JP-A) Nos. 8-29354 and 7-325041 may be effective in achieving such dust particle detection. These dust particle detecting methods project a light beam on the surface of a wafer, focuses a microscope on a spot formed on the surface of the wafer by the light beam, enlarge scattered light by the microscope, and observe (detect) two-dimensionally a region of the surface of the wafer in the field of view of the microscope by a highly sensitive CCD camera or the like disposed at a dark field position and provided with an image intensifier. Since these methods detect haze, i.e., noise attributable to light scattered by the surface of the wafer, two-dimensionally, the S/N ratio of a detection signal obtained by these methods is higher than that of a detection signal provided by a conventional dust particle inspection system which measures the haze by a photomultiplier tube by detecting integrated light scattered from minute disorder of the surface of the wafer
FIG. 5 shows a particle detecting system disclosed in JP-A Nos. 7-325041 and 8-29354. Shown in FIG. 5 are an X-Y stage 1, a workpiece (silicon wafer) 2, an Ar laser 3 for projecting a laser beam on the workpiece 2, a detecting light beam 4 for detecting minute dust particles, reflected light beam 5 reflected by the workpiece 2, a microscope 8 for observing the workpiece 2, a CCD camera 9 with an image intensifier for taking an image of a portion of the workpiece 2 observed by the microscope 8, and a CRT 10 for displaying an image taken by the CCD camera 9. The detecting light beam 4 can be polarized by a polarizing plate 11. FIGS. 6 and 7 illustrate a mode of projecting the detecting light beam 4 on the workpiece. Shown in FIG. 6 and 7 are a dust particle 6 on the workpiece 2, irregularly reflected light 7, and a spot 12 of the detecting light beam 4.
In operation, the silicon wafer 2, i.e., a workpiece, is mounted on the X-Y stage 1. An imaginary coordinate system is set on the silicon wafer 2 on the X-Y stage 1 with reference to a feature in shape of the silicon wafer 2, such as an orientation flat or a notch formed on the silicon wafer 2. A method of setting a coordinate system is described in detail in JP-A No. 7-25118. The detecting light beam 4 is projected on the silicon wafer 2 to form the spot 12 as shown in FIG. 7 on the surface of the silicon wafer 2. An image of the spot 12 magnified by the microscope 8 disposed in a dark field region is taken by the CCD camera 9, and the image of the spot 12 is displayed on the CRT 10 for observation. The microscope 8 is focused on a surface on which the spot 12 is formed. If there are dust particles 6 in the field of view of the microscope 8, i.e., in the spot 12, irregularly reflected light 7 is observed with the X-Y stage 1 at a position represented by coordinates (x, y) (FIG. 7). If any dust particle is not found in the spot 12, the detecting light beam 4 is reflected regularly and the reflected light beam 5 cannot be observed from the dark field position.
In an observation system shown in FIG. 5 and FIG. 7, the field of view A of the microscope 8 disposed at the dark field position includes the spot 12 formed by the detecting light beam 4 on the silicon wafer 2. Since the dust particles 6 in the spot 12 reflect light irregularly in irregularly reflected light 7, the positions of the dust particles 6 can be determined through the observation of the irregularly reflected light 7 by the microscope 8. Experiments proved that the contrast between a portion from which the irregularly reflected light 7 is reflected and a portion from which no irregularly reflected light is reflected is very high, so dust particles of particle sizes of 0.03 .mu.m or below can clearly be identified, and a detection signal of a satisfactorily high S/N ratio can be obtained. If there is no dust particle 6 in the spot 12, the detecting light beam 4 is substantially perfectly regularly, and hence practically nothing can be observed by the microscope 8 disposed at the dark field position. Therefore, the irregularly reflected light 7 reflected by the dust particles 6 can be observed by the microscope disposed at the dark field position even if the detecting light beam 4 forms the spot 12 in a size far greater than that of the dust particles 6 and, consequently, the positions of the dust particles 6 in the spot 12 can easily be determined in a high accuracy.
As described above, the dust particle detecting system is constructed to observe a portion of a workpiece. Therefore, the method employing the CCD camera needs an additional system which enables the observation of the entire surface of the workpiece if the detection of dust particles on the entire surface of the workpiece is desired.